1. Field of the Invention
The present invention relates to a catalyst used for purifying the harmful substances such as hydrocarbons (HC), nitrogen oxides (NOx), carbon monoxide (CO), etc. present in the exhaust gas discharged from an internal combustion engine of automobile or the like.
2. Description of Related Art
Various catalyst techniques have heretofore been proposed for purification of the exhaust gas discharged from an internal combustion engine of automobile or the like. Recently, there have been proposed a number of catalysts comprising not only a noble metal (e.g. Pd) (which is an ordinary catalyst active component) but also an alkaline earth metal or an oxide thereof. For example, in Japanese Patent Application Laid-Open No. 237384/1993, a catalyst is disclosed for exhaust gas purification obtained by supporting, on a monolithic structure, a catalyst active component containing a noble metal (e.g. Pd or Rh), an alkaline earth metal oxide, a lanthanum oxide, a cerium oxide, a zirconium oxide and a heat-resistant inorganic oxide.
In recent years, it has been one of the most important technical tasks to effectively purify the unburnt combustible components (e.g. HC, CO and H2), in particular, HC discharged during the cold start of automobile engine. During the cold start, the engine is usually operated in a rich fuel-to-air ratio and HC is contained in the exhaust gas in a large amount; meanwhile, the catalyst used is still below the operating temperature and its purification ability is low; as a result, a large portion of HC tends to be discharged into the atmosphere without being purified, that is, in an unburnt state. Hence, a technique is desired which can effectively purify the HC present in the exhaust gas discharged during the cold start of engine.
When the above-mentioned prior art is investigated from such a standpoint, the catalyst disclosed in Japanese Patent Application Laid-Open No. 237384/1993 uses an alkaline earth metal oxide in order to increase the Pd purifiability for NOx in a fuel-rich atmosphere; however, the literature pays little attention to the purification of HC and CO in a fuel-rich atmosphere.
The present invention has been completed in view of the above situation. The object of the present invention is to provide a catalyst for exhaust gas purification wherein a particular alkaline earth metal and a noble metal are used as catalyst components, the amounts of these components supported on a carrier are optimized for allowing the catalyst to have improved low-temperature light-off performance and, thereby, the catalyst""s purification ability for, in particular, HC is increased in a low-temperature and fuel-rich atmosphere such as experienced during the cold start of automobile engine.
According to the present invention, there is provided a catalyst for exhaust gas purification capable of purifying the hydrocarbons, nitrogen oxides and carbon monoxide present in the exhaust gas discharged from an internal combustion engine, which catalyst comprises a monolithic carrier and a catalyst layer containing Pd, Ba and a heat-resistant inorganic oxide, formed on the carrier, wherein the amount of Pd supported on monolithic carrier is 100-300 g per ft3 of monolithic carrier (3.53xc3x9710xe2x88x923 to 1.06xc3x9710xe2x88x922 g per cc of monolithic carrier), the amount of Ba supported on monolithic carrier is 0.010-0.060 g per cc of monolithic carrier in terms of BaO, and the weight ratio of Pd and Ba as expressed as BaO is 1:2 to 1:10.
According to the present invention, there is also provided a system for exhaust gas purification comprising:
the above-mentioned catalyst for exhaust gas purification, provided in the exhaust pipe of an internal combustion engine, and
an adsorbent having hydrocarbon adsorbability, also provided in the exhaust pipe.